Optical element having a plurality of decentered reflecting curved surfaces, and optical instrument including the same

ABSTRACT

An integrally formed optical element includes an incident surface into which light flux from an object enters, a plurality of reflecting curved surfaces decentered one to the other from which the light flux entered from the incident surface is reflected and an emergent surface from which the light flux from the reflecting curved surfaces is emitted, wherein an antireflection film is applied onto the incident surface and the emergent surface, and wherein the light flux emitted from the emergent surface forms an image.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element, and moreparticularly, to an optical element suitable for use in an opticalsystem of a video camera, a still video camera and a copying machine.

2. Description of the Related Art

Conventionally, various picture taking optical systems have beenproposed in which reflecting surfaces such as concave mirrors and convexmirrors are utilized.

FIG. 1 schematically illustrates a main part of a so-called mirroroptical system including one concave mirror and one convex mirror.

In the optical system of FIG. 1, light flux 134 from an object isreflected from concave mirror 131, travels toward the object while beingconverged, is reflected from convex mirror 132, and then forms an imageon image surface 133.

The mirror optical system is based on the construction of the so-calledCassegrainian reflecting telescope, and is intended to shorten the totallength of the optical system by folding the optical path of a telephotolens system composed of refracting lenses by the use of two oppositereflecting mirrors.

In addition, in an objective lens system constituting a telescope, anumber of methods in addition to the Cassegrainian method have beenknown in which the total length of the optical system is shortened bythe use of a plurality of reflecting mirrors, based on the principlesdescribed above.

Thus, a method for obtaining a compact optical system has beenconventionally known in which the optical path is efficiently foldedwith use of reflecting mirrors in place of a lens optical system havinga long total length.

However, in the mirror optical system of the Cassegrainian reflectingtelescope and the like, there is a problem that the light from theobject is partially eclipsed by the convex mirror 132.

In order to solve the problem, a mirror optical system has been alsoproposed in which reflecting mirrors are decentered with respect to anoptical axis in order to prevent the other parts of the optical systemfrom shielding the passage region of the light flux 134, that is to say,the principal ray of the light flux is separated from the optical axis135.

FIG. 2 schematically illustrates a main part of the mirror opticalsystem disclosed in U.S. Pat. No. 3,674,334. In this mirror opticalsystem, the center axis itself of the reflecting mirrors is decenteredwith respect to an optical axis to separate the principal ray of thelight flux from the optical axis, thereby solving the problem ofeclipse.

The mirror optical system of FIG. 2 includes concave mirror 111, convexmirror 113 and concave mirror 112 in the order of passage of the lightflux. These mirrors are originally rotary symmetrical with respect tooptical axis 114, as shown by a chain double-dashed line. Only the upperportion of the concave mirror 111 with respect to optical axis 114, onlythe lower portion of convex mirror 113 with respect to the optical axis,and only the lower portion of concave mirror 112 with respect to opticalaxis 114 are used to construct an optical system in which a principalray of light flux from an object is separated from optical axis 114, andthe eclipse of light flux 115 is eliminated.

FIG. 3 schematically illustrates a main part of a mirror optical systemdisclosed in U.S. Pat. No. 5,063,586. In the mirror optical system ofFIG. 3, a center axis of the reflecting mirrors itself is alsodecentered with respect to the optical axis to separate the principalray of the light flux from the optical axis, thereby solving the problemof eclipse.

Referring to FIG. 3, when a vertical axis of object surface 121 isdefined as optical axis 127, center coordinates and center axes (axesconnecting centers of reflecting surfaces and centers of curvaturesthereof) 122a, 123a, 124a and 125a of the reflecting surfaces of convexmirror 122, concave mirror 123, convex mirror 124 and concave mirror 125are decentered with respect to optical axis 127. By suitably setting theamount of decentering and the radius of curvature of each surface, theeclipse of light flux 128 from an object due to reflecting mirrors isprevented, so that an object image is efficiently formed on an imagesurface 126.

These reflecting-type picture taking optical systems contain many partsor components. Thus, in order to obtain a required optical performance,it is necessary that each of the optical parts are accurately assembled.More particularly, according to the picture taking optical system of atype in which reflecting mirrors are decentered with respect to theoptical axis for the prevention of an eclipse of the light ray from theobject, each of the reflecting mirrors must be disposed with differentdecentering amounts. As a result, structures for mounting reflectingmirrors thereto become complicated, and extremely precise mountingaccuracy is required.

As one of the methods for solving the above problems, a method may beconsidered in which assembly error of the optical parts is avoided bycombining mirror systems into one block.

Hitherto, as examples of such mirror systems, there have been opticalprisms such as pentagonal roof prisms and Porro prisms which are usedfor viewfinder systems, and a color separation prism for separating thelight flux from a picture taking lens is separated into three colors ofred, green and blue to form object images based on each color of thelight on the surface of each image pick-up device.

In these optical prisms, since a plurality of reflecting surfaces areintegrated, the reflecting surfaces are placed accurately in relation toone another, so that positions of the reflecting surfaces need not beadjusted.

In these optical prisms, however, there is a problem in that a harmfulghost light associated with an irregular incident light incident frompositions and angles other than those of an effective light ray isfrequently generated.

A function of the pentagonal roof prism which is often used in asingle-lens reflex camera as a typical example of the optical prismswill now be described with reference to FIG. 4. Referring to FIG. 4,there are provided a picture taking lens 101, a quick-return mirror 102,a focal plane 103, a condenser lens 104, the pentagonal roof prism 105,an eyepiece 106, an observer's pupil 107, an optical axis 108 and animage surface 109.

A light flux from an object (not shown) is reflected upward of a camerafrom the quick-return mirror 102 after passing through the picturetaking lens 101 so as to form an image on the focal plane 103 located ata position equivalent to the image surface 109. A condenser lens 104 forforming an exit pupil of the picture taking lens 101 on the observer'spupil is disposed behind the focal plane 103, and the pentagonal roofprism 105 for making an object image on the focal plane 103 into acorrect image is disposed behind the condenser lens 104.

The object light which has entered the incident surface 105a of thepentagonal roof prism 105 is subjected to a reversal of an object imagefrom right to left, and then is emitted to the observer's side as theobject light by the reflecting surface 105c.

The object light emitted to the observer's side by the reflectingsurface 105c passes through an emergent surface 105d of the pentagonalroof prism 105, reaches the eyepiece 106 so as to be formed into asubstantially parallel light by a refracting force of the eyepiece 106,and then reaches the observer's pupil 107 so as to be observed.

In the pentagonal roof prism constructed as described above, a ghostlight shown by an arrow in FIG. 4 incident at an angle different fromthat of an effective light is reflected in the order of the roof surface105b and reflecting surface 105c, is totally reflected from the incidentsurface 105a, and then is emitted from the lower portion of the emergentsurface 105d to the observation side. Since the ghost light differs fromthe normal effective light ray in the number of reflections, an imageturned upside down appears on the lower portion of an observationscreen.

In order to remove the ghost light, a light shielding groove 100 isprovided in the emergent surface 105d of the pentagonal roof prism 105.

In addition, the overall surface of the prism 105 is covered by blackpaint except for the incident surface 105a and the emergent surface105d, whereby a reflecting film to be evaporated onto the roof surface105b and the reflecting surface 105c is protected from environmentalchange, such as a change in temperature and humidity, and further, alight ray from the outside of the prism is shielded.

Furthermore, an optical system has been known in which curvature isgiven to reflecting surfaces of a prism. FIG. 5 schematicallyillustrates a main part of an observation optical system disclosed inU.S. Pat. No. 4,775,217. This observation optical system is intended toobserve scenery of the outer world, and to observe an image displayed ona information display means by superimposing the image on the scenery.

According to this observation optical system, a display light flux 145emitted from an image displayed on an information display means 141 isreflected from a surface 142 toward the outer world, and enters a halfmirror surface 143 having a convex surface. After being reflected fromthe surface of the half mirror surface 143, the display light flux 145is made into a substantially parallel light flux by the concave halfmirror surface 143, and forms an enlarged virtual image of the displayedimage after being refracted and passing through the surface 142, andthen enters an observer's pupil 144 so as to allow the observer torecognize the displayed image.

On the other hand, a light flux 146 from the outer world enters asurface 147 which is substantially parallel to the reflecting surface142, and is refracted therefrom so as to reach the concave half mirrorsurface 143. A semi-transmission film is evaporated onto the concavehalf mirror surface 143, and a part of the light flux 146 passes throughthe concave half mirror surface 143 and enters the observer's pupil 144after being refracted and passing through the surface 142, whereby theobserver visually recognizes the displayed image by superimposing it onthe scenery of the outer world.

FIG. 6 schematically illustrates a main part of an observation opticalsystem disclosed in Japanese Unexamined Patent Publication No. 2-297516.This observation optical system is also intended to observe scenery ofthe outer world, and to observe an image displayed on a informationdisplay means by superimposing the image on the scenery.

According to this observation optical system, a display light flux 154emitted from an information display means 150 passes through a plane 157constituting a prism Pa and enters a parabolic reflecting surface 151.The display light flux 154 is reflected from the reflecting surface 151to become a convergent light flux, and forms an image on a focal plane156. At this time, the display light flux 154, reflected from thereflecting surface 151, has reached the focal plane 156 while beingtotally reflected between two parallel planes 157 and 158 whichconstitute the prism Pa, whereby a totally slim optical system can beobtained.

The display light flux 154 emitted as a divergent light from the focalplane 156 enters a half mirror 152 having a parabolic surface afterbeing totally reflected between the planes 157 and 158, and is reflectedfrom the surface of the half mirror 152. At the same time, the lightflux 154 forms an enlarged virtual image of the displayed image by areflecting force of the half mirror 152 and becomes a substantiallyparallel light flux, and then passes through the surface 157 to enterthe observer's pupil 153, thereby allowing the observer to recognize thedisplayed image.

On the other hand, a light flux 155 from the outer world passes througha surface 158b constituting a prism Pb, the half mirror 152 and thesurface 157, and then enters the observer's pupil 153. The observervirtually recognizes the displayed image by superimposing it on thescenery of the outer world.

The principal function of the conventional optical prism such as thepentagonal roof prism is to reverse an image by changing the directionin which a light ray travels. Therefore, the reflecting surfaces of theoptical prism are commonly formed by planes alone, and the optical prismdoes not impart curvatures to the reflecting surfaces, and positivelycorrect aberration on the reflecting surfaces.

In the observation optical systems disclosed in U.S. Pat. No. 4,775,217and Japanese Unexamined Patent Publication No. 2-297516, asemi-transmission film is used in order to observe a displayed image andrecognize an object image, which reduces a transmission light amount ofthe displayed image. Therefore, as described above, a method is commonlyadopted in which total reflecting surfaces are used in order to minimizethe loss of the light amounts on each of the reflecting surfaces.

The total reflecting surfaces are often formed by planes alone tosimplify construction. From the viewpoint of correcting aberration,however, it is desirable that the reflecting surfaces are also formedinto curved surfaces to optimally correct aberration.

However, when total reflecting conditions are to be satisfied withrespect to all light rays entering the reflecting surfaces, there is nodegree of freedom of the shapes of the reflecting surfaces, so that theaberration is not efficiently corrected on the reflecting surfaces.

SUMMARY OF THE INVENTION

It is an object of the present invention to evaporate an antireflectionfilm onto incident and emergent surfaces of an optical element whichincludes an integrally-formed plurality of reflecting surfaces, eachhaving a curvature for effectively correcting aberration, and tominimize the reduction in a transmission light amount of the overalloptical element.

In order to achieve the above object, an optical element of the presentinvention in which an incident surface into which light flux from anobject enters, a plurality of reflecting curved surfaces which reflectthe incident light entered from the incident surface and are decenteredone to the other, and an emergent surface from which the light flux fromthe plurality of curved surfaces is emitted are integrally formed, ischaracterized in that an antireflection film is applied onto theincident surface and the emergent surface when forming an object imageon a predetermined surface.

More particularly, the optical element of the present inventionpreferably has the following characteristics:

A reflecting film is formed on the reflecting curved surfaces, and therange of formation of the reflecting film is substantially equal to arange where the light flux forming an image on an image forming surfaceenters the reflecting surfaces;

Portions other than the range of formation of the reflecting film arediffusing surfaces;

A protective layer is provided on the reflecting film formed on thereflecting surfaces; and

The protective films are painted black.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a main part of a conventional mirroroptical system;

FIG. 2 schematically illustrates a main part of another conventionalmirror optical system;

FIG. 3 schematically illustrates a main part of still anotherconventional mirror optical system;

FIG. 4 illustrates a conventional pentagonal roof prism;

FIG. 5 schematically illustrates a main part of a conventionalobservation optical system;

FIG. 6 schematically illustrates a main part of another conventionalobservation optical system;

FIG. 7 illustrates a coordinate system which defines data of componentsof an optical element of the present invention;

FIG. 8 schematically illustrates a main part of an optical elementaccording to an embodiment of the present invention;

FIG. 9 illustrates optical paths according to the embodiment of thepresent invention;

FIG. 10 is a light ray distribution chart on a concave refractingsurface R8 according to the embodiment of the present invention;

FIG. 11 is a light ray distribution chart on a convex mirror R9according to the embodiment of the present invention;

FIG. 12 is a light ray distribution chart on a convex mirror R10according to the embodiment of the present invention;

FIG. 13 is a light ray distribution chart on a concave mirror R11according to the embodiment of the present invention;

FIG. 14 is a light ray distribution chart on a convex mirror R12according to the embodiment of the present invention;

FIG. 15 is a light ray distribution chart on a concave mirror R13according to the embodiment of the present invention;

FIG. 16 is a light ray distribution chart on a concave refractingsurface R14 according to the embodiment of the present invention;

FIG. 17 illustrates shapes of the surfaces and distribution of lightrays of an optical element according to the embodiment of the presentinvention viewed from the positive side of a Z-axis; and

FIG. 18 illustrates shapes of the surfaces and distribution of lightrays of an optical element according to the present invention viewedfrom the negative side of the Z-axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before describing the preferred embodiment, various components in theembodiment and common facts through the embodiment will be described.

FIG. 7 illustrates a coordinate system which defines data of componentsof an optical element of the present invention. In the embodiment, thei-th surface along a light ray (which is shown by dashed line in FIG. 7,and is referred to as a reference axis light ray) travelling from anobject to an image surface is defined as the i-th surface.

In FIG. 7, the first surface R1 is a diaphragm, the second surface R2 isa refracting surface (incident surface) which is co-axial with the firstsurface, the third surface R3 is a reflecting surface tilted withrespect to the second surface R2, the fourth and fifth surfaces R4 andR5 are reflecting surfaces which are tilted and shifted with respect tothe front surfaces thereof, respectively, and the sixth surface R6 is arefracting surface (emergent surface) which is shifted and tilted withrespect to the fifth surface R5. The second to sixth surfaces R2 to R6are constructed on one optical element which is formed by a medium, suchas glass and plastic, and illustrated as the optical element 1 in FIG.7. Two refracting surfaces (the incident surface and the emergentsurface) and at least three reflecting surfaces of curved surfaces areintegrally formed in the optical element 1 of the present invention.

Therefore, according to the construction of FIG. 7, a medium from theobject surface (not shown) to the second surface R2 is air, a mediumfrom the second surface R2 to the sixth surface R6 is a certain commonmedium, and a medium from the sixth surface R6 to the seventh surface R7(not shown) is air.

Since the optical system of the present invention is a decenteredoptical system, each of the surfaces constituting the optical system donot have a common optical axis. Thus, in the embodiment of the presentinvention, an absolute coordinate system is first established in which acenter of an effective diameter of the first surface is defined as anorigin.

In addition, in the embodiment of the present invention, the centerpoint of the effective diameter of the first surface is defined as theorigin, and a path of a light ray (the reference axis light ray) passingthrough the origin and the center of a final image forming surface isdefined as a reference axis of the optical system. Further, thereference axis in the embodiment of the present invention has its owndirection. The direction is one in which the reference axis light raytravels for forming an image.

Although the reference axis of the optical system is established asdescribed above, the most convenient axis may be determined as thereference axis from the viewpoint of optical design, arrangement ofeccentric aberration or expression of the shape of each surface of theoptical system. However, the path of the light ray passing through anyone of the diaphragm or entrance pupil, exit pupil, the center of thefirst surface of the optical system and the final image forming surfaceis commonly defined as the reference axis.

That is, in the embodiment of the present invention, the path in whichthe light ray passing through the center point of the effective diameterof the first surface, i.e., diaphragm surface, to the center of thefinal image forming surface is refracted and reflected by refracting andreflecting surfaces is established as the reference axis. The surfacesare arranged in the order in which the reference axis is to be refractedor reflected.

Therefore, the reference axis finally reaches the center of the imagesurface while changing its direction following a law of refraction orreflection in accordance with the set order of the surfaces.

All of the tilted surfaces constituting the optical system of theembodiment of the present invention are basically tilted within the samesurface. Thus, each of the axes of the absolute coordinate system aredefined as follows:

Z-axis: the reference axis passing through the origin, and extendingtoward the second surface R2;

Y-axis: a straight line passing through the origin, and forming 90° in acounterclockwise direction with the Z-axis within the tilted surface(within the surface of FIG. 7); and

X-axis: a straight line passing through the origin, and perpendicular tothe z-axis and y-axis (a straight line perpendicular to the surface ofFIG. 7).

In addition, when expressing the shape of the i-th surface constitutingthe optical system, the shape can be understood more easily byestablishing a local coordinate system in which the intersection of thereference axis and the i-th surface is defined as the origin to expressthe shape of the surface by the local coordinate system, rather than byexpressing the shape of the i-th surface by the absolute coordinatesystem. Therefore, in the embodiment expressing data of components ofthe present invention, the shape of the i-th surface is expressed by thelocal coordinate system.

In addition, a tilt angle within the YZ plane of the i-th surface isexpressed by an angle θi (unit: degree) in which an angle in thecounterclockwise direction with respect to the Z-axis of the absolutecoordinate system is taken as a positive angle. Therefore, in theembodiment of the present invention, the origins of the local coordinatesystems of each of the surfaces are on the YZ plane in FIG. 7. Further,there is no decentering of surfaces within the XZ plane and the XYplane. Still further, the y-axis and z-axis of the local coordinate (x,y, z) of the i-th surface are tilted at the angle θi with respect to theabsolute coordinate system within the YZ plane. More specifically, eachof the axes of the local coordinate system are defined as follows:

z-axis: a straight line passing through the origin of the localcoordinate system, and forming an angle θi in the counterclockwisedirection with the Z-direction of the absolute coordinate system withinthe YZ plane;

y-axis: a straight line passing through the origin of the localcoordinate system, and forming 90° in a counterclockwise direction withthe z-direction within the YZ plane; and

x-axis: a straight line passing through the origin of the localcoordinate system, and perpendicular to the YZ plane.

Accordingly, as shown in FIG. 7, the center point of the effectivediameter of the first surface R1 is labelled (Z1, Y1), and is defined asthe origin (0, 0). The angle θ1=0. Along surface R2, the origin of thelocal coordinate system is identified as (Z2, Y2), and θ2=0. Alongsurface R3, the origin of the local coordinate system is labelled (Z3,Y3), and θ3>0. Likewise, along surface R4, the origin of the localcoordinate system is labelled (Z4, Y4), and θ4>0. Along surface R5, theorigin of the local coordinate system is labelled (Z5, Y5), and θ5>0.Finally, along surface R6, the origin of the local coordinate system islabelled (Z6, Y6), and θ6>0.

In addition, Di (D1-D6 in FIG. 1) is a scaler amount representing aspacing between the origins of the local coordinates of the i-th surfaceand the (i+1)-th surface, and Ndi and vdi are refractive index and Abbenumber of a medium between the i-th surface and the (i+1)-th surface,respectively. As shown in FIG. 7, the refractive indices Nd2 through Nd5are equal to one another. Likewise, Abbe numbers νd2 through νd5 areequal to each other. Refractive indices Nd1 and Nd6=1, as the mediumthere is air.

Optical active surfaces in the embodiment of the present invention areof spherical surfaces and of rotary asymmetrical aspheric surfaces. Aradius of curvature of the spherical surface portion is referred to asr_(i). When the center of curvature is on the side of the first surfacealong the reference axis (dashed line in FIG. 7) travelling from thesurface to the image surface, the sign of the radius of curvature r_(i)is negative, whereas the sign of the same is positive when the center ofcurvature is on the side of the image forming surface.

The spherical surface has the shape represented by the followingexpression (1): ##EQU1##

In addition, the optical system of the present invention includes atleast one rotary asymmetrical aspheric surface, and the shape thereof isrepresented by the following expression (2):

Letting

    A=(a+b)·(y.sup.2 ·cos.sup.2 t+x.sup.2)

    B=2a·b·cos t[1+{(b-a)·y·sin t/2a·b}+[1+{(b-a)·y·sin t/(a·b)}-{y.sup.2 /(a·b)}-

    {4a·b·cos.sup.2 t+(a+b).sup.2 sin.sup.2 t}x.sup.2 /(4a.sup.2 b.sup.2 cos.sup.2 t)].sup.1/2]

wherein A and B are variables; a, b, and t are constant,

    z=A/B+C.sub.02 y.sup.2 +C.sub.20 x.sup.2 +C.sub.03 y.sup.3 +C.sub.21 x.sup.2 y+C.sub.04 y.sup.4 +C.sub.22 x.sup.2 y.sup.2 +C.sub.40 x.sup.4(2)

wherein Cnm is a constant.

Since the above expression includes odd number order terms in relationto x, the curved surface defined by the above expression has a shapesymmetrical with respect to a yz plane. In addition, if the followingcondition is satisfied, the curved surface exhibits the shapesymmetrically with respect to an xz plane:

    C.sub.03 =C.sub.21 =0, and t=0.

Further, if a condition C₀₂ =C₂₀ C₀₄ =C₄₀ =C₂₂ /2 is satisfied, thecurved surface exhibits a rotary symmetrical shape. When the aboveconditions are not satisfied, the curved surface exhibits a rotaryasymmetrical shape.

Incidentally, in the embodiment of the present invention, a conditionC₀₂ =C₂₀ =0 is satisfied, and a high order asymmetrical aspheric shapeis added to a basic shape of the second order curved surface.

In the embodiment of the present invention, as shown in FIG. 7, thefirst surface is a diaphragm. In addition, a horizontal half angle ofview uy means a maximum angle of view of a light flux entering into thediaphragm R1 within the YZ plane of FIG. 7, and a vertical half angle ofview u_(x) means a maximum angle of view of the light flux entering intothe diaphragm R1 within the XZ plane. Further, the diameter of thediaphragm R1 (first surface) is indicated as the diaphragm diameter.This is related to brightness of the optical system. Since the entrancepupil is located on the first surface, the diameter of the diaphragm isequal to the diameter of the entrance pupil.

An effective range of an image on the image surface is indicated as animage size. The image size is represented by a rectangular region inwhich the size in the y-direction is taken as a horizontal size, and thesize in the x-direction is taken as a vertical size.

In the embodiment of the present invention, the size of the opticalsystem is also indicated, which is defined by the effective diameter ofthe first surface.

FIG. 8 schematically illustrates a main part of an optical apparatusaccording to an embodiment of the present invention. The drawing is asectional view within the YZ plane, and also shows an optical path of anaxial light flux. Referring to FIG. 8, an optical element 2 has anintegrally formed plurality of reflecting surfaces each having acurvature. The optical element 2 is composed of, in the order from anobject, a concave refracting surface (incident surface) R8, fivereflecting surfaces of a concave mirror R9, a convex mirror R10, aconcave mirror R11, a convex mirror R12 and a concave mirror R13, and aconvex refracting surface (emergence surface) R14, and the direction ofthe reference axis entering into the optical element 2 and the directionof the reference axis emitted from the optical element 2 are paralleland opposite to each other. Numeral 3 denotes an image pick-up devicesuch as CCD, and R15 denotes a light receiving surface thereof. Adiaphragm 4 (R7) is disposed on the object side of the optical element2, and numeral 5 denotes the reference axis of the optical element 2.Both of the refracting surfaces are formed into a rotary symmetricalspheric shape, and all of the reflecting surfaces are anamorphicsurfaces which are symmetrical with respect to the YZ surface.

Numeric data of this embodiment will now be shown below. In the numericdata, for example, "e-03" represents "multiple by 10⁻³ ". Thisembodiments is directed to a picture taking optical system of ahorizontal angle of view of 63.4° and a vertical angle of view of 49.6°.

Horizontal half angle of view: 31.7°

Vertical half angle of view: 24.8°

Diameter of diaphragm: 2.0 mm

Image size: 4 mm (horizontal)×3 mm (vertical)

Size of optical system

    ______________________________________                                                         θi                                                                              Di                                                     Yi Zi (°) (mm) Ndi νdi                                            ______________________________________                                        R7   0.00    0.00    0.00  1.82  1          diaphragm                           R8 0.00 1.82 0.00 7.49 1.58310 30.20 refracting                                      surface                                                                R9 0.00 9.30 18.49 9.86 1.58310 30.20 reflecting                                     surface                                                                R10 -5.93 1.43 3.23 9.30 1.58310 30.20 reflecting                                    surface                                                                R11 -10.65 9.44 -12.55 8.90 1.58310 30.20 reflecting                                 surface                                                                R12 -11.50 0.58 -22.91 9.39 1.58310 30.20 reflecting                                 surface                                                                R13 -18.82 6.46 -25.63 8.02 1.58310 30.20 reflecting                                 surface                                                                R14 -18.82 -1.56 -0.01 3.68 1  refracting                                            surface                                                                R15 -18.82 -5.24 -0.01 0.00 1  image                                                 surface                                                              ______________________________________                                    

Spherical Shape

    ______________________________________                                                           r.sub.i (mm)                                               ______________________________________                                                R7           ∞                                                    R8 -7.648                                                                     R14 10.757                                                                    R15 ∞                                                                 ______________________________________                                    

Aspherical Shape

    ______________________________________                                        R9                                                                              a = -1.09716e + 01 b = -1.25390e + 01 t = 2.15145e + 01                       C.sub.02 = 0. C.sub.20 = 0.                                                   C.sub.03 = 6.87152e - 05 C.sub.21 = -1.21962e - 04                            C.sub.04 = 3.59209e - 05 C.sub.22 = 1.02173e - 04 C.sub.40 = 4.95588e -                                05                                                   R10                                                                           a = -2.34468e + 00 b = 4.88786e + 00 t = -3.56094 + 01                        C.sub.02 = 0. C.sub.20 = 0.                                                   C.sub.03 = -4.48049e - 03 C.sub.21 = -7.45433e - 03                           C.sub.04 = 1.81003e - 03 C.sub.22 = 2.09229e - 03 C.sub.40 = -8.28024e                                 - 04                                                 R11                                                                           a = -6.11985e + 00 b = 1.70396e + 01 t = -2.17033e + 01                       C.sub.02 = 0. C.sub.20 = 0.                                                   C.sub.03 = -3.23467e - 04 C.sub.21 = -1.07985e - 03                           C.sub.04 = -3.70249e - 05 C.sub.22 = -1.74689e - 04 C.sub.40 = -1.21908e                                - 04                                                R12                                                                           a = ∞ b = ∞ t = 0.                                                C.sub.02 = 0. C.sub.20 = 0.                                                   C.sub.03 = 1.10097e - 03 C.sub.21 = -3.73963e - 04                            C.sub.04 = -1.59596e - 04 C.sub.22 = -3.22152e - 04 C.sub.40 = -1.74291e                                - 04                                                R13                                                                           a = -2.11332e + 01 b = -1.31315e + 03 t = 1.70335e + 00                       C.sub.02 = 0. C.sub.20 = 0.                                                   C.sub.03 = 8.29145e - 05 C.sub.21 = -1.11374e - 03                            C.sub.04 = -2.50522e - 05 C.sub.22 = -5.28330e - 05 C.sub.40 = -2.91711e                                - 05                                              ______________________________________                                    

An image forming operation of this embodiment will now be described. Anincident light amount of axial light flux 6 entering along the referenceaxis 5 is regulated by the diaphragm 4 (R7), and then enters the concaverefracting surface R8 of the optical element 2. The light flux 6 whichhas entered the concave refracting surface R8 is emitted as a divergentlight flux by a power of the concave refracting surface R8, is reflectedfrom the concave mirror R9, and primarily forms an object image on anintermediate image forming surface N1 by a power of the concave mirrorR9. By forming the object image within the optical element 2 in theearly stage as described above, an increase in the effective diametersof the surfaces disposed closer to the image side than to the diaphragm4 is retarded.

The light flux 6 which has primarily formed the image on theintermediate image forming surface N1 reaches the convex refractingsurface R14 while being reflected from the convex mirror R10, concavemirror R11, convex mirror R12 and concave mirror R13 to undergo opticalactions due to the powers of the reflecting mirrors, and then the lightflux 6 is refracted by the power of the convex refracting surface R14 soas to form an object image on the light receiving surface R15.

Thus, in the optical element 2, the light flux reaches the lightreceiving surface R15 by repeating refraction by the incident andemergent surfaces, and reflection by a plurality of reflecting mirrorseach having a curvature, so that the optical element 2 serves as a lensunit having desired optical performance and a positive power overall.

In this embodiment, the direction of the reference axis entering theoptical element 2 and that emitting therefrom are parallel and oppositeto each other. In addition, all of the reference axes including theincident reference axis and emergent reference axis are given in FIG. 8(on the YZ plane).

In this embodiment, an antireflection film is evaporated onto therefracting surfaces of the optical element 1 constructed as describedabove, and a reflecting film is evaporated onto a suitable range of thereflecting surfaces of the same so as to minimize a reduction intransmission light amount of the whole optical element.

This matter will be described. The transmission light amount of theoverall optical element can be generally represented by the followingexpression:

Transmission light amount={(reflectance of the reflectingsurface)**(number of reflecting surfaces)}×{(transmittance of thetransmission surface)**(number of transmission surfaces)}

wherein the power exponent**represents the number of reflectingsurfaces.

In this embodiment, five reflecting surfaces and two transmissionsurfaces are included. For example, if the reflectance of the reflectingsurface is 95%, and the transmittance of the transmission surface is98%, the transmission light amount in this embodiment is obtained by thefollowing expression:

    Transmission light amount=0.95.sup.5 ×0.98.sup.2 =0.74

Thus, the ratio of the emergent light amount to the incident lightamount is 74%.

If it is required to increase the above transmission light amountfurther, the reflecting surfaces are set so as to satisfy a totalreflection condition while optimizing the shapes of some reflectingsurfaces.

FIG. 9 illustrates optical paths of this embodiment. FIG. 10 to 16 aredistribution charts (light rays distribution charts) each showing adistribution of incident points of the incident light entering into eachof the surfaces obtained when tracing light rays with respect to axialpoint and abaxial points in this embodiment. Incidentally, the light raywhich forms an image on the light receiving surface R9 is hereinafterreferred to as an image forming light ray.

FIG. 10 is a light ray distribution chart of the incident points of thelight ray on the concave refracting surface R8, shown in FIG. 9. In FIG.10, the incident points are substantially distributed within a circlewith reflecting the shape of the diaphragm 4 (R7). FIG. 14 is a lightrays distribution chart of the incident points on the convex mirror R12near a pupil image forming surface. In FIG. 14, the incident points aresubstantially distributed within a circle reflecting the shape of thepupil. On the other hand, FIGS. 13 and 15 are light ray distributioncharts of the incident points on the concave mirrors R11 and R13 apartfrom the pupil image forming surface, respectively. In these drawings,the incident points are distributed within an ellipse deviated from thecircular shape.

FIG. 12 is a light ray distribution chart of the incident points on theconvex mirror R10 near the primary image forming surface, and FIG. 16 isa light ray distribution chart of the incident points on the convexrefracting surface R14. In these drawings, the incident points aredistributed within a rectangle reflecting the shape of the lightreceiving surface R15. FIG. 11 is a light ray distribution chart of theincident points on the concave mirror R9. In FIG. 11, the incidentpoints are distributed within a trapezoid.

Thus, the optical element 2 includes the integrally-formed reflectingsurfaces and refracting surfaces each exhibiting different shape of thelight ray distribution thereon.

FIGS. 17 and 18 illustrate the optical element 2 of this embodimentviewed from the positive side and negative side of the Z-axis direction,respectively. These drawings also illustrate light ray distributions ofeach of the surfaces, respectively.

As shown in FIG. 17, the optical element 2 is constructed so that theconcave refracting surface R8, the convex mirrors R10 and R12 which areincident surfaces, and the convex refracting surface R14 which is theemergent surface, are adjacent on to the other.

In addition, as shown in FIG. 18, the optical element 2 is constructedso that the concave mirrors R9, R11 and R13 are adjacent one to other.

In consideration of an adjacent portion between the concave mirrors R9and R11 of FIG. 18, the image forming light rays are distributed in atrapezoidal shape on the concave mirror R9, but in an elliptical shapeon the concave mirror R11. In contrast, each of the reflecting surfacesof the optical element 2 are formed into rectangular shapes from aconstruction viewpoint. Therefore, when the reflecting film is formed(evaporated) onto the full range of the reflecting surfaces, thereflecting film would be formed onto a range of the reflecting surfacesexcept for the incident range of the image forming light rays.

In consideration of a ghost light generated in the optical element 2,the ghost light may be usually generated with respect to the incidentlight from positions and angles different from those of the imageforming light rays. Therefore, the light rays generally impinge on thepositions of each of the surfaces departing from the incident range ofthe image forming light rays.

When the reflecting film is formed on the overall reflecting surfaces,the risk of generating ghost light from a portion other than theincident range of the image forming rays is remarkably increased.Therefore, it is desirable that the range of formation of the reflectingfilm is as small as possible to avoid generating ghost light.

Thus, in this embodiment, the reflecting film is formed on thereflecting mirror surfaces of R9 to R13 only within the range shown bythe dotted lines, i.e., within the range which is substantially the sameas the range in which the image forming light rays enter the reflectingsurfaces (of course, with a certain allowance), so that ghost light isnot generated at portions other than the incident range of the imageforming light rays.

The optical element of this embodiment has at least three, integrallyformed reflecting curved surfaces, ghost light is particularly easilygenerated. However, according to the present invention, the range offormation of the reflecting film is optimized as described above so asto effectively prevent ghost light.

In addition, according to the present invention, diagonally shadedportions in FIGS. 17 and 18 are coarse surfaces (uneven surfaces), suchas diffusing surfaces, so that the intensity of unnecessary light orghost light entering the portions can be reduced by a diffusion actionof the diffusing surfaces.

Generally, when a metallic film such as an aluminum or silver film isdirectly evaporated on the reflecting surfaces of the optical element,it is highly possible that the reflecting surfaces will corrode underthe environmental condition such as the high temperature and highhumidity, whereby it becomes difficult to secure durability of thereflecting surfaces.

Thus, in the embodiment of the present invention, after evaporating thereflecting film such as an aluminum or silver film, and a dielectricfilm onto the reflecting surfaces, a protective film is evaporatedthereon. As the protective film, a substance, such as SiO₂ which isresistant to environmental change, may be evaporated onto the reflectingsurfaces. However, in order to impart a light shielding action to thereflecting surfaces, black paint and the like may be preferably appliedonto the reflecting surfaces.

With the arrangement described above, in which the optical elementincludes a plurality of integrally formed reflecting surfaces, eachhaving a curvature for effectively correcting aberration, with anantireflection film evaporated onto incident and emergent surfacesthereof, the range of evaporation of a reflecting film is optimized whenevaporating the reflecting film onto the reflecting surfaces, and theform of the portions the reflecting surfaces other than the range ofevaporation of the reflecting film are defined. As a result, there canbe provided an optical element which minimizes the reduction in thetransmission light amount of the entire optical element, and reduces theintensity of ghost light generated within the optical element.

In addition, an optical element having at least one of the followingadvantages can be provided:

The reflecting film is evaporated onto a plurality of reflectingsurfaces each having a curvature, and the antireflection film isevaporated onto transmission surfaces, thereby minimizing the reductionin the transmission light amount of the overall optical element;

The reflecting film is evaporated in accordance with the range in whichthe image forming light ray enters the reflecting surfaces, therebypreventing generation of the ghost light;

The portions of the reflecting surfaces, except for the range in whichthe image forming light ray enters the reflecting surfaces, are madecoarse to prevent ghost light in the above range;

The reflecting film is evaporated in the shape of a circle, an ellipse,or a polygon, thereby preventing the generation of the ghost light;

After evaporating the reflecting film onto the reflecting surfaces, aprotective layer is provided thereon, thereby obtaining an opticalelement resistant to environmental change;

Light shielding properties are given to the protective layer, therebypreventing the generation of ghost light; and

When total reflecting surfaces are included in the optical element, theyare effectively used to prevent reduction in the transmission lightamount.

While the present invention has been described with respect to what ispresently considered to be the preferred embodiment, it is to beunderstood that the invention is not limited to the disclosedembodiment. To the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. An integrally formed optical element,comprising:an incident surface into which light flux from an objectenters; a plurality of reflecting curved surfaces decentered one to theother from which the light flux entered from said incident surface isreflected, at least one of said plurality of reflecting curved surfacesbeing formed into a rotationally asymmetrical aspheric shape, whereinsaid shape has only one plane of symmetry; and an emergent surface fromwhich the light flux from said plurality of reflecting curved surfacesis emitted, wherein an antireflection film is applied onto at least oneof said incident surface and said emergent surface, and wherein thelight flux emitted from said emergent surface forms an image.
 2. Anoptical element according to claim 1, said plurality of reflectingcurved surfaces having a reflecting film formed thereon, wherein a rangeof formation of said reflecting film is substantially equal to a rangein which an effective light flux from the object enters.
 3. An opticalelement according to claim 2, said plurality of reflecting curvedsurfaces having diffusing surfaces formed on the portions thereof otherthan the range of formation of said reflecting film.
 4. An opticalelement according to claim 2, said plurality of reflecting curvedsurfaces having a reflection protective layer formed on said reflectingfilm.
 5. An optical element according to claim 4, wherein a color ofsaid reflection protective layer is black.
 6. An optical instrumentcomprising an integrally formed optical element, said optical elementcomprising:an incident surface into which light flux from an objectenters; a plurality of reflecting curved surfaces decentered one to theother from which the light flux entered from said incident surface isreflected, at least one of said plurality of reflecting curved surfacesbeing formed into a rotationally asymmetrical aspheric shape, whereinsaid shape has only one plane of symmetry; and an emergent surface fromwhich the light flux from said plurality of reflecting curved surfacesis emitted, wherein an antireflection film is applied onto at least oneof said incident surface and said emergent surface, and wherein thelight flux emitted from said emergent surface forms an image.
 7. Anoptical instrument according to claim 6, said plurality of reflectingcurved surfaces having a reflecting film formed thereon, andwherein arange of formation of said reflecting film is substantially equal to arange in which an effective light flux from the object enters.
 8. Anoptical instrument according to claim 7, said plurality of reflectingcurved surfaces having diffusing surfaces formed on the portions thereofother than the range of formation of said reflecting film.
 9. An opticalinstrument according to claim 7, said plurality of reflecting curvedsurfaces having a reflection protective layer formed on said reflectingfilm.
 10. An optical instrument according to claim 9, wherein a color ofsaid reflection protective layer is black.
 11. An optical instrumentaccording to claim 6, further comprising an image pick-up means forpicking-up an image formed by said optical element.
 12. An opticalelement according to claim 1, wherein an antireflection film is appliedonto said incident surface and an antireflection film is applied ontosaid emergent surface.
 13. An optical instrument according to claim 6,wherein an antireflection film is applied onto said incident surface andan antireflection film is applied onto said emergent surface.